An ideal voltage reference is an electronic device that ideally produces a fixed voltage irrespective of the loading on the device, power supply variation and temperature. Voltage references are critical components in both analog and digital systems. The accuracy, temperature sensitivity and drift of references impact the performance of many circuit blocks such as analog-to-digital converters, digital-to-analog converters and power management circuitry. The ever decreasing scale of transistors now requires lower reference voltages, such as below 1 volt. Additionally, these voltage references must have low temperature sensitivity and high initial accuracy.
One of the most commonly used conventional voltages references in integrated circuits is the bandgap voltage reference. A bandgap-based reference uses analog circuits to add a multiple of the voltage difference between two bipolar junctions biased at different current densities to the voltage developed across a diode. The diode voltage has a negative temperature coefficient (i.e. it decreases with increasing temperature), and the junction voltage difference has a positive temperature coefficient. When added in the proportion required to make these coefficients cancel out, the resultant constant value is a voltage equal to the bandgap voltage of the semiconductor. In silicon, the bandgap voltages is approximately 1.25V. Typically, the bandgap reference is designed to achieve a first order temperature cancellation that gives a zero temperature coefficient at a particular temperature.
Although sufficient for some applications, the conventional bandgap voltage reference restricts the reference voltage to that of the energy bandgap of silicon (1.25V). Therefore, the conventional bandgap voltage reference is insufficient for applications requiring a reference voltage below 1 volt (“sub-1V”).